1. Field of the Invention
The invention relates generally to liquid pump systems and, more particularly, to circuitry for controlling a variable-speed motor drive thereof.
2. Description of the Related Art
Domestic water systems typically include a line-operated (i.e., 60 Hz, single-phase power) motor for driving a pump-motor assembly to retrieve water from a well. The pump-motor assembly is generally submerged in the well at the end of a drop pipe. A less efficient alternative configuration is a surface-mounted jet pump and submerged ejector nozzle.
To maintain an approximately constant supply pressure, these water systems also typically include a pressurized storage tank and a pressure sensing switch (hereinafter "pressure switch") that causes the pump-motor assembly to run when pressure is low (i.e., when the tank water level is low) and stop when pressure is high (i.e., when the tank water level is high). However, starting single-phase motors is inefficient and cannot be repeated too frequently or the motor will overheat. As a result, typical domestic water systems use a relatively large pressure tank (e.g., 20 gallons) and a relatively large pressure differential (e.g., 20 psi) to limit the frequency of motor starting.
Recent progress in power electronics has resulted in the incorporation of a frequency changer (i.e., variable-speed motor drive) in the pump-motor assembly. Frequency control has allowed the motor to be operated at higher speeds than the 3450 rpm typical of 60 Hz line-operated motors, in turn allowing the pump to be physically smaller. For submersible pumps, this advantage is realized in a reduction in the number of stages, inasmuch as the size of each stage in a multi-stage submersible pump is restricted by the well bore diameter. Another advantage of a variable-speed pump-motor assembly involves "soft starting," or ramping up the speed of the motor during starting to provide a more efficient startup procedure.
In one such variable-speed motor-pump assembly, the variable-speed motor drive was included as part of a submerged pump-motor assembly unit. A pressure switch located near the pressure tank was utilized to cut power to the pump-motor assembly once the pressure reached a predetermined high level. Cutting power to the pump-motor assembly, however, would also de-energize the variable-speed motor drive. As a result, a capacitor bank in the variable-speed motor drive had to be recharged during every startup. Recharging the capacitor bank, particularly if done quickly, places extreme stresses on the rectifiers that supply current to the capacitor bank. Reducing the recharge rate to avoid damage to the rectifier, however, undesirably decreases the responsiveness of the pump-motor assembly. A lower responsiveness translates into a larger tank.
Another variable-speed system similarly cut the power to the pump-motor assembly via a control unit disposed near the tank. In this system, a pressure switch near the control unit coupled a pair of signal conductors to the control unit. These signal conductors then led to the variable-speed motor drive via a cable separate from the power conductors carrying the 60 Hz power.
Other variable-speed systems have replaced the on/off signal of a pressure switch with an analog control signal developed by a pressure transducer. The pressure transducer signal has also been delivered as a digital signal to the embedded microcontroller, which can then decide to start, stop, or adjust: the speed of the motor. Generally, these systems relying upon a pressure transducer have often severely limited the closed loop bandwidth in order to maintain system stability. This bandwidth limitation significantly decreases the responsiveness of the system. Other systems have had to utilize a flow sensor in addition to the transducer to determine when the pump-motor assembly should completely shut down.